1. Field of the Invention
The present invention relates to a transparent toner for forming a transparent toner image formed on a recording medium with a color toner image and more particularly to improvements in transparent toner useful in electrophotography adapted to be transferred and fixed onto or around a color toner image which is desired to be provided with gloss such as photographic image by electrophotography, developer including the transparent toner, gloss-providing unit and image forming device.
2. Background Art
In order to form a color image on the surface of a recording medium or make a color duplicate using a color image forming device capable of forming a color image by an electrophotographic process, electrostatic recording process or the like, it has been hereto fore practiced to execute the following image forming steps.
In some detail, a color original is irradiated with light beam. The light beam reflected by the color original is then color-separated and read by a color scanner. The data thus read is then subjected to predetermined image processing or color correction by an image processor to give a plurality of color image signals according to which a semiconductor laser or the like is then modulated to emit laser beams modulated by the image signals. The surface of an image carrier made of an inorganic photoreceptor such as selenium and amorphous silicon or an organic photoreceptor including a charge-generating layer made of a phthalocyanine dye, bisazo pigment or the like is irradiated with these laser beams by a plurality of times for each color to form a plurality of electrostatic latent images. These electrostatic latent images are then sequentially developed with four color toners of yellow (Y), magenta (M), cyan (C) and black (K). The toner images thus developed are then transferred from the image carrier made of an inorganic or organic photoreceptor onto a recording medium such as paper on which they are then fixed by, e.g., a heat-pressing process fixing unit. In this manner, a color image is formed on the surface of the recording medium.
While the color image thus formed is smoothened on the surface thereof during heat fixing and thus has some gloss, the paper which is a recording medium normally has no gloss. Thus, the color image has a glossiness different from that of the paper. It is also known that some kinds of the binder resin to be incorporated in the color toners or some heat fixing processes cause the toners to change in its viscosity during heat fixing, resulting in the change of glossiness of the color image, as disclosed in JP-A-5-142963, JP-A-3-2765, JP-A-63-259575, JP-5-158364, JP-A-2001-222138, JP-A-11-249339, JP-A-2002-287426 and JP-A-2003-167380.
The tastes in the glossiness of color image differ widely with the kind of images, purpose, etc. In the case of photographic originals such as person and scenery, high gloss images prevail in people's tastes from the standpoint of sharpness in image quality.
As techniques for obtaining a high gloss image by a color image forming device there have been already proposed techniques in Patent References 1 to 3, etc. According to these references, the use of a color copying machine with properly selected toners, fixing conditions, etc. makes it possible to obtain a high gloss image.
In accordance with these proposed techniques, the glossiness of the image area formed by the toners can be raised, but the glossiness of the non-image area cannot be raised, making it impossible to uniformalize the glossiness of the surface of the recording medium. These techniques are also disadvantageous in that an uneven surface of color toners remains on the surface of the image, making it impossible to attain smoothness as in silver salt system photograph or print and hence give a smooth texture.
Further, JP-5-158364 discloses a device capable of heat-melting a recording medium having a color toner image and a transparent toner image formed thereon by a belt type fixing unit and then cooling and peeling the fixing unit from the recording medium to form an image having a high gloss as attained in silver salt system photograph.
However, the aforementioned device is disadvantageous in that there occurs a prominent step on the border of high density are a with low density area. In particular, there occurs a depression like a hole at a small spot of low density in high density area. This phenomenon is attributed to the fact that the binder resin in the transparent toner is not fluid enough to fill the step in the color toner image. This phenomenon becomes remarkable when the recording medium passes through the fixing unit at a high speed. Thus, the above cited techniques are disadvantageous in that both the requirements for high printing speed and high gloss and uniformity in image cannot be attained at the same time so far as the fixing unit is used under practical temperature and pressure conditions.
Moreover, the transparent toner to be used in the above cited techniques is disadvantageous in that the transparent toner layer thus fixed undergoes durability troubles such as deformation and offset under high temperature and humidity conditions or after prolonged storage.
In other words, taking into account the reduction of energy consumption by image making, low temperature fixability is essential. In order to satisfy the desired low temperature fixability, it is an effective solution to reduce the molecular weight of the resin and lower the glass transition point of the resin.
On the other hand, there is an apprehension that an image having a smooth surface like a photograph is subject to blocking (bonded so firmly that the two sheets cannot be peeled off each other or, if peeled, the surface of image is damaged) when stored in automobile or warehouse in summer time or allowed to stand at high temperature as in transportation at the ship bottom while being superposed on the surface or back surface of another image or on material of album.
In this case, in order to improve durability at high temperature, i.e., heat resistance, it is effective to raise the glass transition point and the molecular weight of the resin.
Further, the enhancement of toughness against bending of image, i.e., mechanical strength of image, too, is an important assignment. In order to enhance mechanical strength, it is an effective solution to raise the molecular weight of the resin.
Thus, the enhancement of mechanical strength and heat resistance is contrary to the enhancement of low temperature fixability. In particular, in order to make an image having a high gloss as in silver salt system photograph, it is necessary to further raise the fixing temperature. Therefore, it is more difficult to satisfy all the three requirements at the same time.
With the recent demand for binder resin having an excellent low temperature fixability and a good preservability, the use of a crystalline polyester resin as disclosed in JP-A-2003-167380 or the combined use of a crystalline polyester resin and an amorphous polyester resin as disclosed in Patent References 5 to 7 has been studied. These approaches are considered to be an effective technique for accomplishing both low temperature fixability and heat resistance and durability against offset and blocking. When these techniques are applied to transparent toner, both the low temperature fixability and durability can be observed enhanced. However, the resulting fixed image becomes cloudy due to crystal dispersion structure (spherulite dispersion structure) characteristic to crystalline polyester resin and thus loses sharpness. These techniques are also disadvantageous in that the resulting image undergoes embrittlement and gloss change due to slow progress of crystallization over an extended period of time.
The related art transparent toner including an amorphous resin is also disadvantageous in that it has a low mechanical strength against bending and thus easily undergoes cracking. The transparent toner including a crystalline resin stays flexible after fixed but can undergo cracking more easily than the transparent toner including an amorphous resin due to the effect of crystal interface when time elapses until crystallization proceeds.
An image including a photographic color toner image and a transparent toner image has a high bulk of toner and thus undergoes a high stress when given a bending mechanical force. Thus, such an image undergoes cracking even when given a small external force. Cracks on a uniform glossy surface are very prominent and thus drastically the value of print.